Self-stabilizing combustion apparatus



Filed Nov. 29, 1963 All? INVENTOR. JDHN l7. NESBIT T.

United States Patent SELF-STABILIZING (IQMEUSTIQN APPARATUS John D.Nesbitt, Toledo, Ohio, assignor to Midland-Ross Corporation, Toledo,Ohio, a corporation of Uhio Filed Nov. 29, 1963, Ser. No. 326,895 9Claims. (Cl. 1581.5)

This invention relates to an improvement in self-stabilizing combustionapparatus of the type described in Canadian Patent 651,907.

In the aforesaid Canadian patent there is described combustion apparatuswhich is self-stabilizing, i.e., apparatus which may be utilized in lowtemperature service without the need for a separate pilot burner.According to the teachings of this patent a portion of the stream offuel, which is introduced axially into the combustion apparatus, isdrawn backwards toward the inlet end of the apparatus by virtue of thenegative pressure resulting from the tangential admission of a portionof the combustion air requirements to the apparatus at a locus adjacentthe inlet end thereof. There is then formed a combustible air-fuelmixture at the interface of the spinning air stream and the portion ofthe fuel that is drawn backwards toward the inlet end of the apparatus.This airfuel mixture is readily ignitable and, when once ignited, thecombustion thereof proceeds with remarkable stability thereby serving topilot the combustion of the remaining portion of the fuel stream. In apopular embodiment of the invention of the aforesaid Canadian patent ahigh degree of the combustion of the remainder of the fuel stream ismade to occur within the combustion apparatus itself by virtue of thegenerally radial admission of additional combustion air by means of aplurality of ports in the wall means which serves to define thecombustion chamber of the apparatus. Thus, such combustion devices areusually referred to as combustors.

In the operation of combustors of the aforesaid type there have beennoted two problems during periods of sustained operation at relativelylow inputs, solutions to which it is the object of this presentinvention to provide. First, at relatively low fuel inputs the forwardvelocity of the fuel introduced through the fuel inlet means hecomesquite small with the result that an increased quantity of fuel is drawnbackwards toward the inlet end of the combustor. When this happens theair-fuel mixture which forms between the spinning air stream and thebackwardly flowing fuel stream becomes excessively rich which leads toincomplete combustion of the backwardly flowing fuel stream with theconsequent formation of undesirable deposits of carbon on portions ofthe combustor.

Second, in many instances it is desired to provide any of several commontypes of flame supervising means to take corrective action in the eventthe flame of the combustor becomes extinguished. When a combustor of theaforesaid type, equipped with flame supervising means, is operated atrelatively low fuel inputs, the rate of heat release is frequently solow that the flame supervising means cannot detect the presence offlame. This is troublesome in that it may lead to unnecessary andundesirable shutdowns of the oven or other device with which thecombustor is associated.

For a further understanding of the invention, attention is directed tothe following portion of the specification, the drawing and the appendedclaims.

In the drawing:

FIG. 1 is an elevational view, in section, of a combustor embodying thepresent invention; and

PEG 2 is a sectional view taken on line 22 of FIG. 1.

A combustor in accordance with the present invention is shown generallyat 11 and comprises wall means 12 forming a combustion chamber 13 whichis substantially 3,221,795 Patented Dec. 7, 1965 circular in transversesection and which has an inlet end and an outlet end shown,respectively, at 14 and 15. The inlet end 14 of combustion chamber 13 issubstantially closed by backplate 16 which is disposed transversely ofand preferably, as shown, perpendicular to the axis of combustionchamber 13. An annular air plenum 17 is formed surrounding combustionchamber 13 by means of cylindrical wall 18 in combination with theperipheral portion of backplate 16, an annular frontplate 19', and wallmeans 12. Air for combustion is delivered to plenum 17 by meanscomprising a flanged inlet connection 21. Inlet connection 21 is shownas comprising a transversely disposed multiported orifice plate 22 toimpart a controlled pressure drop to the supply of combustion air to aidin accurately metering and controlling its rate of delivery. Fuel forcombustion is introduced into combustion chamber 13 proximate to inletend 14 by fuel inlet means comprising a pipe 23 which terminates a shortdistance downstream of backplate and which is disposed substantiallycoaxial to chamber 13.

A stream of air, equal to about 410% of the amount required forstoichiometric combustion of the maximum rate of fuel delivery throughfuel pipe 23, is introduced tangentially to combustion chamber 13 at alocus closely adjacent inlet end 14 thereof by air inlet meanscomprising one or more pipes 24 whose axis is directed parallel to aplane disposed generally transversely t0 and preferably substantiallyperpendicular to the axis of combustion chamber 13 (e.g., the plane ofbackplate 16). In this manner the air which is introduced through pipe24 will have not more than a very small component of motion directedtoward the outlet end 15 of combustion chamber 13. The tight spinningmotion of the air introduced through pipe 24 will create a region ofreduced pressure surrounding fuel pipe which will draw a portion of thefuel from pipe 23 backwardly toward backplate 1.6. A readily ignitablefuel-air mixture will form adjacent backplate 16 between the portion ofthe fuel from pipe 23 that is drawn backwardly toward backplate 16 atthe interface between it and the spinning stream of air from air pipe.When once ignited, either by hand or by means such as a spark plug 25,this fuel-air mixture which is being continually generated will burnwith great stability to pilot or stabilize the combustion of theremainder of the fuel stream introduced into combustion chamber 13 bymeans of fuel pipe 23.

In the illustrated embodiment a large degree of the combustion of theremainder of the fuel stream from fuel pipe 23 is made to occur withincombustion chamber 13 itself by providing wall means 12 with a pluralityof ports 26 to admit air from plenum 17 radially into combustion chamber13. Ports 26 are preferably arranged in a series of axial stages and areso sized and spaced as to provide combustion air in amounts, and atlocations, which will result in sustaining the combustion of theremaining portion of the fuel stream at the maximum rate which will notlead to flame quench mg.

In the operation of combustors of this description for prolonged periodsat rates of fuel introduction substantially below the maximum designcapacity of the combustor it has been noted that carbon deposits areformed on backplate 16. Carbon deposits are generally undesirable inthat they may lead to difiiculty in re-igniting the combustor and inthat they may interfere with means provided to monitor the presence ofcombustor flame, e.g., means comprising a flame rod R in the illustratedembodiment. Additionally, the formation of carbon deposits isparticularly undesirable in combustors for certain applications such ascombustors for supplying heated air for paint drying and food drying orroasting. In such applications there is the danger that deposited carbonmay flake off and pass into contact with the product and thereby detractfrom its quality.

The reason for the formation of carbon deposits on the backplate ofcombustors of the foregoing type at low delivery rates is believed to bethat the rate of fuel drawn toward the backplate increases as the totalrate of fuel delivery decreases due to the fact that the fuel stream hasless forward momentum by virtue of its reduced velocity. In line withthis reasoning, then, it is believed that fuel-air mixture which formsat the backplate becomes excessively rich at low fuel deliveryrateswhich leads to the formation of free carbon in that portion of thereaction zone in contact with backplate 16 with resultant accumulationof carbon particles on backplate 16 (assuming, of course, that thecombustor is fired with a carbonaceous fuel, e.g., natural gas).

That the foregoing analysis of the problem is correct is believed tohave been established by a solution which has been demonstrated andwhose efficacy is analytically consistent with the analysis of theproblem. This solution resides in the tangential introduction of astream of air into fuel pipe 23 at an upstream locus. This isaccomplished by providing fuel inlet pipe 23 with a tangential air inletpipe 27 whose axis is directed transversely of a plane through the axisof fuel inlet pipe 23 and which is so oriented as to cause the airdelivered therethrough to rotate in the same direction as the airdelivered through pipe(s) 24. The delivery of air to pipe 27 may beaccomplished by an air line 28 which bleeds air from flanged air inletconnection 21 of combustor 11 through a bleed connection 29. The rate ofair delivery through tangential pipe 27 is preferably small. As aspecific example, without being intended as a limitation on theinvention, it has been found that the delivery of air at a rate of 220c.f.h. (cubic feet per hour) has been of considerable utility incombating carbon formation at reduced fuel delivery rates in a combustorsized for a maximum rate of fuel delivery of 2000 c.f.h.

It is believed that the tangential admission of air to fuel inlet pipe23 is effective in combating carbon deposition on backplate 16 at lowdelivery rates because the spinning air is preferentially drawn towardthe backplate due to its centrifugal momentum thereby reducing thetendency to draw fuel toward the backplate and, additionally, byproviding a layer of air adjacent the backplate intermediate the layerof fuel thereby to insure that the mixture which forms adjacent thebackplate will not be so rich as to lead to the formation of carbondeposits.

Because of the fact that the tangential delivery of air to fuel inletpipe 23 through tangential air inlet pipe 27 is of no particular valueat relatively high fuel delivery rates there may be provided means, suchas a solenoid valve 31., to terminate the delivery of air through line28 when the delivery of fuel through line 32, as determined by flowmeter 33, exceeds a predetermined value.

Another problem connected with the operation of combustors according tothe foregoing description at fuel delivery rates substantially belowmaximum is that of maintaining a locus of flame adjacent the inlet endof the combustor that is sufficiently intense to be detected by flamemonitoring means such as means comprising flame rod R. Where the flameis of only marginal intensity the flame monitoring means may, from timeto time, not be able to detect the presence of flame and may,unnecessarily, cause the oven, air heater, or the like with which thecombustor is associated to be shut off. It has been found thattangential admission of air to fuel nozzle 23 through tangential inletpipe 27 is of no particular benefit in eliminating such nuisanceshutdowns.

It has been found, however, that the stabilizing flame at the inlet end14 of combustion chamber 13 can be substantially intensified, thereby toavoid nuisance shutdowns of flame-monitored combustors, by introducingan annular, spinning, partially aerated stream of fuel into combustionchamber 13 at a locus very closely adjacent backplate 16. This isaccomplished in the illustrated embodiment by providing means comprisinga pipe 34 surrounding a portion of fuel inlet pipe 23 and forming,therewith, an annular chamber 35 which opens into combustion chamber 13.Pipe 34 is provided with a tangentially disposed inlet 36 whose axis isdisposed in a plane substantially perpendicular to the axis ofcombustion chamber 13 and which is so oriented as to cause the partiallyaerated fuel which is delivered therethrough to rotate in the samedirection as the air delivered through pipe(s) 24. A partially aeratedstream of fuel is delivered to inlet 36 by means of a line 37 frommixing means such as a T connection 38 which receives fuel from a line39 and air from a branch line 41 from line 23.

In the operation of a combustor comprising such a feature it isdesirable that the mass rate of flow of both fuel and air to chamber 35not be greater than necessary to adequately intensify the stabilizingflame. By way of example only, it has been found, in connection with acombustor fired with natural gas, that the delivery of fuel through line39 to mixing T 38 at a rate of the order of 2% of the maximum rate offuel delivery through fuel pipe 23, when coupled with the delivery ofair through line 41 to mixing T 38 at a volumetric rate equal from 2 to3 times the rate of fuel flow to mixing T 38, will result insubstantially intensified stabilizing flame conditions as noted by thesubstantially reduced number of nuisance shutdowns of flame-monitoredcombustors.

The best mode known to me to carry out this invention has been describedabove in terms sufiiciently full, clear, concise, and exact as to enableany person skilled in the art to make and use the same. It is to beunderstood, however, that it is contemplated that other modes ofpracticing the invention can be made by a skilled artisan withoutdeparting from the scope of the invention which is defined only by theappended claims.

I claim:

1. In combustion apparatus comprising, in combination: wall meansforming a combustion chamber, said wall means comprising a generallycylindrical wall having a corresponding inside and outside surface andthe ends of which cylindrical wall define an inlet and exit end of thecombustion chamber, and a backplate aifixed to the cylindrical wall atthe inlet end thereof; a fuel delivery pipe having an inlet and exit endand a corresponding inside and outside surface and extending through thebackplate into said combustion chamber substantially along the axis ofthe cylindrical wall and terminating at a point intermediate said inletand exit ends of said combustion chamber for introducing a stream offuel sub stantially axial to said combustion chamber; and primary airinlet means connected to the outside surface of said cylindrical walladjacent said backplate, substantially tangential to the inside surfaceof said cylindrical Wall, and in a plane disposed substantiallyperpendicular to the axis of the chamber for introducing a firstspinning stream of air comprising a portion of the combustion airrequirements of the fuel stream; the improvement comprising secondaryair inlet means connected to the outside surface of said fuel deliverypipe, intermediate its inlet and exit ends, substantially tangential tothe inside surface of the fuel delivery pipe in a direction forintroducing to said fuel delivery pipe and hence to said combustionchamber a second stream of air circumposing the fuel stream and spinningin the same direction as the first stream of air.

2. Apparatus according to claim 1 wherein the generally cylindrical wallof said combustion chamber diverges outwardly from a point intermediateits inlet and exit ends to substantially the exit end thereof.

3. Apparatus according to claim 2 wherein said cylindrical wall meanshas a plurality of ports at least in the diverging portion thereof andfurther comprising means for delivering a plurality of radial streams ofair to the combustion chamber through the ports to sustain combustion ofthe fuel stream.

4. In combustion apparatus comprising, in combination: wall meansforming a combustion chamber, said wall means comprising a generallycylindrical wall having a corresponding inside and outside surface andthe ends of which cylindrical wall define an inlet and exit end of thecombustion chamber, and a backplate aflixed to the cylindrical wall atthe inlet end thereof; a fuel delivery pipe having an inlet and exit endextending through the backplate into said combustion chambersubstantially along the axis of the cylindrical wall and terminating ata point intermediate said inlet and exit ends of said combustion chamberfor introducing a stream of fuel substantially axial to said combustionchamber; and primary air inlet means connected to the outside surface ofsaid cylindrical Wall adjacent said backplate, substantially tangentialto the inside surface of said cylindrical Wall, and in a plane disposedsubstantially perpendicular to the axis of the chamber for introducing aspinning stream of primary air comprising a portion of the combustionair requirements of the fuel stream; the improvement comprising meansfor introducing an annular stream of fuel mixed with a portion of itscombustion air through said back plate and into said. chamber adjacentthe inlet end thereof, said stream of fuel and air circumposing saidfuel stream and spinning in the same direction as said primary air.

5. Apparatus according to claim 4 wherein the generally cylindrical wallof said combustion chamber diverges outwardly from a point intermediateits inlet and exit ends to substantially the exit end thereof.

6. In combustion apparatus comprising, in combination: first wall meansforming a combustion chamber, said wall means comprising a generallycylindrical wall having a corresponding inside and outside surface andthe ends of which cylindrical wall define an inlet and exit end of thecombustion chamber, and a backplate affixed to the cylindrical wall atthe inlet end thereof; a fuel delivery pipe having an inlet and exit endextending through the backplate into said combustion chambersubstantially along the axis of the cylindrical wall and terminating ata point intermediate said inlet and exit ends of said combustion chamberfor introducing a stream of primary fuel substantially axial to saidcombustion chamber; and primary air inlet means connected to the outsidesurface of said cylindrical wall adjacent said backplate, substantiallytangential to the inside surface of said cylindrical Wall, and in aplane disposed substantially perpendicular to the axis of the chamberfor introducing a spinning stream of primary air comprising a portion ofthe combustion air requirements of the fuel stream; the improvementcomprising: second Wall means circumposing and forming with said fueldelivery pipe an annular chamber, closed at its upstream end and in opencommunication with said combustion chamber through said backplate; andmeans for introducing secondary fuel and introducing secondary air,sufficient to partially aerate said secondary fuel, to said annularchamber in a manner to deliver a stream of mixed secondary fuel andsecondary air into said chamber ad jacent the inlet end thereof and saidstream of secondary air and fuel circumposing said primary fuel streamand spinning in the same direction as said primary air.

7. Apparatus according to claim 6 wherein said means for introducingsecondary fuel and secondary air to said annular chamber comprises amixture pipe having an inlet and exit end wherein a mixture of secondaryfuel and secondary air is admitted to inlet end of said mixture pipe,and the exit end of said mixture pipe is in communication with saidannular chamber and arranged to introduce said mixture tangentially intothe annular chamber in a plane perpendicular to the axis of said annularchamber.

8. Apparatus according to claim 7 wherein said second wall meanscomprises conduit means coaxial with and circumposing said fuel deliverypipe in a manner to form said annular chamber therebetween, said conduitmeans extending from a point upstream of said backplate and terminatingin communication with the inlet of said com bustion chamber, and whereinthe exit end of mixture pipe is aflixed to said conduit means in a planeperpendicular to the axis of said annular chamber tangent to saidconduit means for introducing said mixture tangentially into saidannular chamber.

9. Apparatus according to claim 6 wherein the generally cylindrical wallof said combustion chamber diverges outwardly from a point intermediateits inlet and exit ends to substantially the exit end thereof; andwherein said cylindrical wall means has a plurality of ports at least inthe diverging portion thereof, and further comprising means fordelivering a plurality of radial streams of air to the combustionchamber through the ports to sustain combustion of the fuel stream.

References Cited by the Examiner UNITED STATES PATENTS 1,073,463 9/1913Banes -22 X 1,763,047 6/1930 Hepburn 158-106 X 1,870,026 8/ 1932Rosencrants 110-22 2,051,099 8/1936 Munford 126-91 2,952,307 9/1960Schramm et al 158-7 3,002,819 10/1961 Brace et a1. 158-76 X FOREIGNPATENTS 1,079,506 5/1954 France.

FREDERICK L. MATTESON, JR., Primary Examiner.

MEYER PERLIN, JAMES W. WESTHAVER,

Examiners.

E. G. FAVORS, Assistant Examiner.

1. IN COMBUSTION APPARATUS COMPRISING, IN COMBINATION: WALL MEANSFORMING A COMBUSTION CHAMBER, SAID WALL MEANS COMPRISING A GENERALLYCYLINDRICAL WALL HAVING A CORRESPONDING INSIDE AND OUTSIDE SURFACE ANDTHE ENDS OF WHICH CYLINDRICAL WALL DEFINE AN INLET AND EXIT END OF THECOMBUSTION CHAMBER, AND A BACKPLATE AFFIXED TO THE CYLINDRICAL WALL ATTHE INLET END THEREOF; A FUEL DELIVERY PIPE HAVING AN INLET AND EXIT ENDAND A CORRESPONDING INSIDE AND OUTSIDE SURFACE AND EXTENDING THROUGH THEBACKPLATE INTO SAID COMBUSTION CHAMBER SUBSTANTIALLY ALONG THE AXIS OFTHE CYLINDRICAL WALL AND TERMINATING AT A POINT INTERMEDIATE SAID INLETAND EXIT ENDS OF SAID COMBUSTION CHAMBER FOR INTRODUCING A STREAM OFFUEL SUBSTANTIALLY AXIAL TO SAID COMBUSTION CHAMBER; AND PRIMARY AIRINLET MEANS CONNECTED TO THE OUTSIDE SURFACE OF SAID CYLINDRICAL WALLADJACENT SAID BACKPLATE, SUBSTANTIALLY ANGENTIAL TO THE INSIDE SURFACEOF SAID CYLINDRICAL WALL, AND IN A PLANE DISPOSED SUBSTANTIALLYPERPENDICULAR TO THE AXIS OF THE CHAMBER FOR INTRODUCING A FIRSTSPINNING STREAM OF AIR COMPRISING A PORTION OF THE COMBUSTION AIRREQUIREMENTS OF THE FUEL STREAM; THE IMPROVEMENT COMPRISING SECONDARYAIR INLET MEANS CONNECTED TO THE OUTSIDE SURFACE OF SAID FUEL DELIVERYPIPE, INTERMEDIATE ITS INLET AND EXIT ENDS, SUBSTANTIALLY TANGENTIAL TOTHE INSIDE SURFACE OF THE FUEL DELIVERY PIPE IN A DIRECTION FORINTRODUCING TO SAID FUEL DELIVERY PIPE AND HENCE TO SAID COMBUSTIONCHAMBER AND A SECOND STREAM OF AIR CIRCUMPOSING THE FUEL STREAM ANDSPINNING IN THE SAME DIRECTION AS THE FIRST STREAM OF AIR.